In the manufacture of paper, it is customary to dry paper to a relatively low moisture content in order to achieve a uniform CD (cross direction) moisture profile. This is true despite the fact that it is known that paper having a relatively high moisture content will fold better, finish easier and give less trouble from curling than paper of lower moisture content. Moreover, printing paper having a high initial moisture content, when conditioned at normal press room humidity, comes to equilibrium by loss of moisture (with a desirably higher moisture content) than paper of lower moisture content which comes to equilibrium by absorption of moisture. However, in order to overcome moisture profile problems originating at the wet end, press section and drier sections of a papermachine, it is common practice to dry the rawstock to a relatively low moisture content. Thus, as regards the paper industry, the controlled and uniform application of moisture to rawstock and/or coated webs is a goal of great interest, and by the present invention, such a goal has been achieved.
Several methods have been proposed in the prior art for applying moisture to a moving sheet or web. Examples of these include roll applicators such as the Dahlgren-type systems, electrostatic spray systems, steam shower devices and water doctor systems particularly as disclosed in the following prior U.S. Pat. Nos.: 3,379,170, 3,776,471, 3,873,025, 3,467,541, 3,735,929, 3,922,129, 3,625,743, 3,782,330.
In general, however, the methods disclosed in the above noted patents involve the use of expensive, complicated and unreliable equipment without any means for achieving a uniform CD moisture profile and/or for the controlled application of small quantities of moisture.
Other examples for adding moisture to a web include, a method and apparatus for applying steam to and condensing moisture on a web that is backed by a heat conducting body as disclosed in U.S. Pat. No. 2,370,811, and a device similar to that disclosed herein that directs a humid atmosphere toward the web, as disclosed in U.S. Pat. No. 3,238,635. However, a careful review of the latter mentioned patents will show that in one case, condensation of steam vapor on the web is produced because the web is backed by a heat sink or metal roll through which a cooling fluid is passed. Meanwhile, in the other patent, where the humid atmosphere directed toward the web is derived from steam, the final mixture applied to the web is so complete that the steam loses all identity as such in the humid atmosphere, and, the emitted premixture is not impelled or jetted adjacent to the web.
In contrast to the methods and apparatus disclosed in the above noted prior art, applicants' invention utilizes a method whereby steam in a substantially dry state is emitted under pressure and at a velocity greater than that of a traveling web into the region between a web and a steamfoil nozzle device. The emitted steam produces a low pressure region between the traveling web and the steamfoil and disrupts the boundary layer air associated with the traveling web to permit intimate contact of the steam with the web. When the steam mixes with the boundary layer air associated with the traveling web, condensation in and on the web occurs to add moisture to the web.
In this respect, applicants' invention relies on principles previously disclosed in connection with the use of airfoil type web drying systems for use in drying paper webs. The latter principles are fully disclosed in the following prior U.S. Pat. Nos. and publications: 3,587,177, 3,711,960, 3,650,043, 3,629,952
TAPPI, April 1973, Vol. 56, No. 4, pp. 86-89 PA0 TAPPI, June 1974, Vol. 57, No. 6, pp. 105-107 PA0 TAPPI, April 1976, Vol. 59, No. 4, pp. 92-96
Accordingly, it may be seen from the above noted disclosures that although the general operating principles behind applicants' invention have been previously proven, applicants are the first to utilize those principles with the application of steam to a web to add moisture to the web.